diff --git a/claudetool/codereview/differential_test.go b/claudetool/codereview/differential_test.go
new file mode 100644
index 0000000..827bf63
--- /dev/null
+++ b/claudetool/codereview/differential_test.go
@@ -0,0 +1,180 @@
+package codereview
+
+import (
+	"testing"
+	"time"
+)
+
+// TestCompareTestResults_NilPointerPanic tests the fix for the nil pointer panic
+// that occurred when comparing test results between commits where a package
+// exists in the after state but not in the before state.
+func TestCompareTestResults_NilPointerPanic(t *testing.T) {
+	// Create a CodeReviewer instance (minimal setup)
+	reviewer := &CodeReviewer{
+		sketchBaseRef: "main",
+	}
+
+	// Create test results where:
+	// - before: empty (no packages)
+	// - after: has a package with tests
+	beforeResults := []testJSON{} // Empty - no packages existed before
+
+	afterResults := []testJSON{
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/newpkg",
+			Action:  "run",
+			Test:    "TestNewFunction",
+		},
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/newpkg",
+			Action:  "pass",
+			Test:    "TestNewFunction",
+			Elapsed: 0.001,
+		},
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/newpkg",
+			Action:  "pass",
+			Test:    "", // package-level pass
+			Elapsed: 0.001,
+		},
+	}
+
+	// This should not panic - before the fix, this would cause a nil pointer dereference
+	// when trying to access beforeResult.TestStatus[test] where beforeResult is nil
+	regressions, err := reviewer.compareTestResults(beforeResults, afterResults)
+	if err != nil {
+		t.Fatalf("compareTestResults failed: %v", err)
+	}
+
+	// We expect no regressions since the new test is passing
+	if len(regressions) != 0 {
+		t.Errorf("Expected no regressions for passing new test, got %d", len(regressions))
+	}
+}
+
+// TestCompareTestResults_NilPointerPanic_FailingTest tests the same scenario
+// but with a failing test to ensure we properly detect regressions
+func TestCompareTestResults_NilPointerPanic_FailingTest(t *testing.T) {
+	reviewer := &CodeReviewer{
+		sketchBaseRef: "main",
+	}
+
+	beforeResults := []testJSON{} // Empty - no packages existed before
+
+	afterResults := []testJSON{
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/newpkg",
+			Action:  "run",
+			Test:    "TestNewFunction",
+		},
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/newpkg",
+			Action:  "fail",
+			Test:    "TestNewFunction",
+			Elapsed: 0.001,
+		},
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/newpkg",
+			Action:  "fail",
+			Test:    "", // package-level fail
+			Elapsed: 0.001,
+		},
+	}
+
+	// This should not panic and should detect the regression
+	regressions, err := reviewer.compareTestResults(beforeResults, afterResults)
+	if err != nil {
+		t.Fatalf("compareTestResults failed: %v", err)
+	}
+
+	// We expect 1 regression for the failing new test
+	if len(regressions) != 1 {
+		t.Errorf("Expected 1 regression for failing new test, got %d", len(regressions))
+	}
+
+	if len(regressions) > 0 {
+		regression := regressions[0]
+		if regression.Package != "sketch.dev/newpkg" {
+			t.Errorf("Expected package 'sketch.dev/newpkg', got %q", regression.Package)
+		}
+		if regression.Test != "TestNewFunction" {
+			t.Errorf("Expected test 'TestNewFunction', got %q", regression.Test)
+		}
+		if regression.BeforeStatus != testStatusUnknown {
+			t.Errorf("Expected before status testStatusUnknown, got %v", regression.BeforeStatus)
+		}
+		if regression.AfterStatus != testStatusFail {
+			t.Errorf("Expected after status testStatusFail, got %v", regression.AfterStatus)
+		}
+	}
+}
+
+// TestCompareTestResults_ExistingPackage tests the normal case where
+// a package exists in both before and after states
+func TestCompareTestResults_ExistingPackage(t *testing.T) {
+	reviewer := &CodeReviewer{
+		sketchBaseRef: "main",
+	}
+
+	// Package exists in both before and after
+	beforeResults := []testJSON{
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/existing",
+			Action:  "pass",
+			Test:    "TestExisting",
+			Elapsed: 0.001,
+		},
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/existing",
+			Action:  "pass",
+			Test:    "",
+			Elapsed: 0.001,
+		},
+	}
+
+	afterResults := []testJSON{
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/existing",
+			Action:  "fail",
+			Test:    "TestExisting",
+			Elapsed: 0.001,
+		},
+		{
+			Time:    time.Now(),
+			Package: "sketch.dev/existing",
+			Action:  "fail",
+			Test:    "",
+			Elapsed: 0.001,
+		},
+	}
+
+	// This should detect the regression from pass to fail
+	regressions, err := reviewer.compareTestResults(beforeResults, afterResults)
+	if err != nil {
+		t.Fatalf("compareTestResults failed: %v", err)
+	}
+
+	// We expect 1 regression
+	if len(regressions) != 1 {
+		t.Errorf("Expected 1 regression, got %d", len(regressions))
+	}
+
+	if len(regressions) > 0 {
+		regression := regressions[0]
+		if regression.BeforeStatus != testStatusPass {
+			t.Errorf("Expected before status testStatusPass, got %v", regression.BeforeStatus)
+		}
+		if regression.AfterStatus != testStatusFail {
+			t.Errorf("Expected after status testStatusFail, got %v", regression.AfterStatus)
+		}
+	}
+}